Method of constantly restoring an electrode during plasma treatment of materials

ABSTRACT

A method of electric arc treatment of materials by constantly restored solid and hollow electrodes in plasma-forming mixtures including hydrocarbons and carbon oxides, whereby carbon is deposited on an initial electrode (2) to form said constantly restored true-carbon electrode (1). Treatment is performed at a temperature of contact of the initial and true electrodes (2,1) not exceeding the temperature of carbon sublimation.

FIELD OF THE INVENTION

The present invention relates to electric welding and metallurgy and,more particularly, to a method of electric arc treatment of materialswith a constantly restored electrode.

BACKGROUND OF THE INVENTION

At present, one of the essential problems faced be welding andmetallurgical technologies is enhancing the stability of electrodes ofelectric arc apparatus operated for welding and similar processes oftreatment of metals, as well as for melting and similar processes oftreatment of metal and slag melts in electric-arc furnaces.

Known in the art is a method of electric arc treatment of materials(U.S. Pat. No. 3,307,011, Cl. 219-74, 1963), providing for enhancing thestability of electrodes, graphite electrodes included, of electric arcapparatus. The method includes introducing into the space between theelectrodes of an electric arc apparatus plasma-forming mixturesincluding carbon-containing compounds chosen from the class ofhydrocarbons and carbon monoxide, and gases inert with respect to thematerial of the electrodes. With the electric arc burning, thesecarbon-containing gases decompose and release free carbon which isdeposited on at least one of the electrodes. Used as gases inert withrespect to particular materials are argon or helium when the electrodeis made of graphite, or else air or nitrogen when the electrode is madeof copper.

This method makes it possible to reduce the erosion of electrodes andthus to extent their operating life with stable arcing at currents from400 to 1000 amperes.

However, this process would not ensure an equilibrium of the respectivequantities of the loss of carbon and its deposition on the workingsurface of the electrode. The weight of the electrode would eithercontinuously diminish, although at a small rate, which means that theelectrode erodes, or else continuously grows, which means that thedimensions of the electrode increase. In the first-mentioned case theelectrode becomes ultimately destroyed. In the second-mentioned case thestability of arcing is affected on account of hindered localization ofthe electrode-adjoining region of the arc on the increased workingsurface of the electrode.

The aforementioned phenomenon can be explained by the following reasons:firstly, the use of only the gases inert with respect to the material ofthe electrodes in the composition of a plasma-forming mixture; secondly,the lack of quantitative relationship between the arc current value, thecomposition and flow rate of the plasma-forming mixture; and thirdly,the lack of relationship between the variation of the composition of theplasma-forming mixture and the time of arcing, particularly, in theinitial period of ignition of the arc. The necessity of using electrodeswhich are either block-shaped or hollow in some cases restrains theapplicability of the method, as it would not ensure the requiredlocalization of either the arc or the jet of plasma within the treatmentarea.

It is for these reasons that the aforementioned method has not yet foundindustrial applications notwithstanding the fact that it was publishedas many as twenty years ago.

There is further known a method of electric arc or plasma treatment ofmaterials (U.S. Pat. No. 4,317,984, Cl. 219-75, 1982; FR, B, No.2431240, Int. Cl.³ HO5H 1/48, B23K 9/16, 1983) consisting of introducinginto the space between the electrodes of an electric arc apparatus aplasma-forming mixture including carbon-containing compounds and anoxidizing agent.

The oxidizing agent is introduced within a time interval correspondingto the reduction of the flow of heat into the electrode on which carbonis deposited from the peak value to a steady value. Depending on thenature of the oxidizing agent, its content in the plasma-forming mixtureis taken to be either 0.4 to 0.9 or 1.05 to 2.5 by volume of thatcorresponding to theoretically complete conversion of hydrocarbons inthe mixture. The first-mentioned range applies to an oxidizing agent inthe form of either oxygen or air, and the second-mentioned range appliesto an oxidizing agent in the form of carbon dioxide. Depending on thechemical affinity of the oxidizing agent for the carbon of thecarbon-containing compound, the latter is supplied into the spacebetween the electrodes in the amount of from (0.5×10⁻³)/n to (6×10⁻³)/nl/amp-sec, where "n" is a number of atoms of carbon in the employedcarbon-containing compound. The carbon-containing compound and oxidizingagent may be introduced into the space between the electrodes eitherseparately or jointly. When the heat flow into the electrode on whichcarbon is deposited attains the peak value already at the moment ofstriking an arc, the oxidizing agent is introduced into the spacebetween the electrodes jointly with the carbon-containing gas prior tostriking an arc.

This method provides for performance of the electrode in an equilibriumof the loss of carbon therefrom and supply of carbon thereto, i.e. in amode of constant restoration of the electrode from the plasma-formingmedium.

However, this method determines all the conditions for ensuring theoperation of the electrode in a mode of constant restoration only on thepart of the plasma-forming medium, and does not deal with similarconditions on the part of the electrode itself. This has been found toimpede the reproducibility of a mode of constant restoration of theelectrode when the value of the current of the arc varied within arelatively broad range, resulting in erosion of the electrode.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of electricarc treatment of materials with a constantly restored electrode,ensuring stable reproducibility of a mode of constant restoration of theelectrode owing to an improved technology of conducting the process inthe space between the electrodes and on the surface of the electrodeitself.

This and other objects are attained in a method of electric arctreatment of materials with constantly restored solid and hollowelectrodes by introducing into the space between these electrodes aplasma-forming mixture including carbon-containing compounds chosen fromthe class of hydrocarbons and carbon oxides to provide in the course ofarc burning carbon deposition upon at least on one of the initialelectrodes made of either carbon or carbide-forming metals, andformation on the surface of said electrode of a true carbon cathode,wherein according to the invention the treatment is conducted at atemperature of contact of the initial and true electrodes, not exceedingthe temperature of sublimation of carbon.

It is expedient to conduct the treatment with the initial solidelectrode with the current density at the initial electrode beingmaintained within a range from 10⁴ A/cm² to 10⁵ A/cm². Electric arctreatment of materials with a constantly restored electrode at atemperature of the contact of the initial and true electrode notexceeding the temperature of sublimation of carbon provides for stablereproducibility of a mode of constant restoration of the electrodewithin a broad range of variation of the parameters of the electric arc,yielding enhanced stability of the electrodes of electric arc devicesused in welding technologies and in metallurgy.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

The invention will be further described in connection with itsembodiments and examples of performing the method in accordance with theinvention, with reference being made to the accompanying schematicdrawing illustrating the method of constant restoration of an electrode,according to the invention.

PREFERRED EMBODIMENT OF THE INVENTION

The method of electric arc treatment of materials with solid and hollowelectrodes resides in the following.

There is introduced into the space between the electrodes of an electricarc device a plasma-forming mixture including carbon-containingcompounds. The latter are hydrocarbons either in their initial gaseousstate (e.g. natural gases of various compositions, such as methane,acetylene and the like) or in a liquefied state (propane, butane, theirblends and the like), and also carbon oxides: its monoxide CO anddioxide CO₂. With an electric arc burning in a gaseous atmosphereincluding the carbon-containing compounds, there is ensured depositionof carbon evolved by decomposition of these compounds in the arc uponthe working surface of at least one of the initial electrodes andformation of this surface of a true carbon electrode constantly restoredfrom the plasma-forming mixture.

The initial electrode, either solid or hollow, is made of either carbon(preferably in the form of graphite) or carbide-forming metals, e.g.hafnium, titanium, zirconium and the like. The treatment is conducted ata temperature of the contact of the initial solid or hollow electrodewith the true constantly restored carbon electrode not exceeding thetemperature of sublimation of carbon, which is about 4000 K. Thetreatment with the initial solid electrode is conducted at the currentdensity at the initial electrode being within a range from 10⁴ A/cm² to10⁵ A/cm².

Generally, the operation of the electrode in a mode of constantrestoration is provided for under a condition that the rate of loss G₁of the material of the electrode does not exceed the rate of depositionG₂ thereupon of the same material from the outside, i.e. G₁ ≦G₂.

Thus, with the electrode operating as the cathode, the loss G₁ of itsmaterial is mainly defined by its evaporation, while the deposition G₂is defined by the deposition of the material in the form of positivelycharged ions which are subsequently neutralized on the working surfaceof the cathode.

Constant restoration of the electrode (e.g. cathode) is possible fromany gaseous atmosphere (the plasma-forming medium) containing compoundsdecomposing at arc temperature, evolving products capable of beingdeposited onto the working surface of the electrode and possessing theappropriate thermal emission and thermophysical characteristics. Theready availability, relatively low cost, ease of transportation, simpleand safe performance of such carbon containing compounds as hydrocarbonsand carbon oxides (its monoxide CO and dioxide CO₂) make them an obviouschoice as the atmosphere ensuring constant restoration of the electrode.Decomposition of these compounds in an electric arc yields, alongside ofother components, carbon featuring both a high point of phase transition(the temperature of sublimation t_(s) ≃4000 K.) and relatively low workon escaping (φ=4.7 eV).

Let us consider the appended schematic drawing of a constantly restoredelectrode on an electric arc device with he arc burning in aplasma-forming medium including carbon-containing compounds.

The true electrode (cathode) 1 of a diameter d₁ is deposited in a thinlayer on the surface of the initially mounted solid bar-shaped electrode2 having a diameter d₂ and on a portion of a copper water-cooling holder3, adjoining this surface. To speed up the formation of the trueelectrode 1 and enhance its adhesion to the initial electrode 2 thelatter is made of either graphite or carbide-forming metal. The initialelectrode 2 may be either solid or hollow, either press-fitted into thecopper water-cooling holder or free.

The initial electrode 2 acts as the working electrode exclusively withinan initial period upon the first striking of an arc, until the trueelectrode 1 is formed of carbon deposited from the carbon-containinggaseous atmosphere of the arc. Further on, the initial electrode 2becomes a passive element functioning, same as the copper water-cooledholder 3, solely as one of the links of the heat transfer chain from theworking surface of the true electrode 1 of the cooling medium.

The abovesaid defines the major and fundamental difference between aconstantly restored electrode and a conventional electrode which wouldalways erode to this or that degree and where the electrode (cathode)region of the arc is always and exclusively localized on a surfaceformed either by the material of the initial electrode 2 (e.g. W or Moin argon) or by compounds of this material with components of thegaseous atmosphere (e.g. Hf, Zr, Ti in air). Therefore, the diameter d₂of the initial electrode 2 in case of conventional electrodes is alwayseither equal to or greater than the diameter of the working surfacevisited by the electrode-adjoining region of the arc. As researchconducted by the authors of the present invention has proved, thediameter d₁ of the true electrode 1 under the conditions of its constantrestoration in most cases is greater than the diameter d₂ of the initialelectrode 2. Thus, there is formed on an electrode including the copperholder 3 with the initial electrode 2--a solid graphite bar of adiameter d₂ =0.15 cm, with an arc burning at 500 A, a true graphiteelectrode of a diameter d₁ of some 0.3 cm, which is constantly restoredfrom the gaseous phase.

Considering that constant restoration of an electrode is feasible onlywhen the supply of the material from the gaseous phase onto the workingsurface of the true electrode 1 is balanced with the loss of thismaterial on account of evaporation, there exist two essentialprerequisites for operation of the elecrode in this mode.

The first one of these conditions defining the content of depositablecarbon in the gaseous phase and has been set fourth, e.g. in the abovecited U.S. Pat. No. 4,317,984 and FR, B, No. 2431240.

The second condition deals with the transfer of heat from the workingsurface of the electrode to a medium cooling this electrode. Thiscondition is concerned with the temperature of the initial electrode 2in the contact with the true electrode 1, with the arc burning at apredetermined current, not exceeding the point of phase transition(either melting or sublimation) of the deposited material, which in caseof graphite is equal to about 4000 K.

The distribution of temperatures in the composition electrode whereinthe working surface of the true electrode 1 receives a heat flow Qtransferred via the initial electrode 2 and copper holder 3 to thecooling medium is generally determined by known relationships of heattransfer through a laminated wall: ##EQU1## where

T₁ is the temperature on the working surface of the true electrode 1, K;

T₂ is the temperature at the interface of the holder 3 and the coolingmedium, K;

F₂ is the surface of heat input into the true electrode 1, m² ;

q is the density of the heat flow into the true electrode, W/m² ;

R_(i) =S_(i) /λ_(i) is the thermal resistance of each link of heattransfer from the working surface of the true electrode 1 to the coolingmedium, m².K.W⁻¹ ;

S_(i) is the extent of each link of heat transfer, m;

λ_(i) is the thermal conductivity of the material of each respectivelink of the heat transfer chain, W.m⁻¹.K⁻¹.

Thus, for a solid initial electrode 2, R=d₂ /2λ, where d₂ is thediameter of the initial electrode 2.

The thermal conductivity of copper (394 W.m⁻¹.K⁻¹) is significantlyhigher than that of graphite (˜60 W.m⁻¹.K⁻¹) and of carbide-formingmetals: hafnium (21 W.m⁻¹.K⁻¹), zirconium (17 W.m⁻¹.K⁻¹), titanium (17W.m⁻¹.K⁻¹), and others.

Therefore, there two directions of reducing the temperature of theinitial electrode 2 in the contact with (at the interface with) the trueelectrode 1.

The first direction is that of reducing the thermal resistance of theinitial electrode 2, which is feasible only by decreasing the diameterd₂ of this electrode, this with a predetermined current value istantamount to stepping up the current density at the electrode.

Table 1 below submits data obtained by calculations and provedexperimentally on temperatures of the contact of the initial electrode 2and the true electrode 1 of an arc burning at 500 A in a mixture ofcarbon dioxide and natural fuel gas, depending on the diameter of andcurrent density at the initial graphite electrode.

                  TABLE 1    ______________________________________             Current    Temperature    Diameter of             density at at interface    initial  initial    of initial 2    electrode             electrode  and true 1    d.sub.2  4 I/πd.sup.2                        electrodes    cm       A/cm.sup.2 K          Remarks    ______________________________________    0.15     2.83 × 10.sup.4                        3605       Electrode (cathode)                                   operates in constant                                   restoration mode    0.20     1.59 × 10.sup.4                        3919       Electrode (cathode)                                   operates in constant                                   restoration mode    0.26     9.42 × 10.sup.3                        4126       Electrode (cathode)                                   is being evoded    0.30     7.07 × 10.sup.3                        4510       Electrode (cathode)                                   is being actively                                   evoded    0.50     2.55 × 10.sup.3                        4915       Electrode (cathode)                                   is being actively                                   evoded    ______________________________________

An analysis of the above data indicates that with current densities atthe initial electrode 2 being short of 10⁴ A/cm², i.e. with its diameterd₂ being above the critical value for 500 A current, the electrode(cathode) is destroyed by the temperature of the contact of the initialand true electrodes 2 and 1 being above the point of sublimation ofgraphite ˜4000 K. Similar results have been obtained for all otherinvestigated current values.

Thus, the current density at the initial electrode 2, equalling 10⁴A/cm², determines at a given current value the critical diameter of theinitial electrode 2 whose increase (i.e. an increase of the diameter d₂)would lead to the destruction of the electrode by the temperature of thecontact of the initial and true electrodes 2 and 1 exceeding thesublimation point of graphite.

When the current density exceeds 10⁵ A/cm², the relatively small area ofcontact of the true electrode 1 with the initial electrode 2 has beenfound to affect the strength of their bonding. This results inmechanical destruction of the electrode on account of delamination ofthe true electrode 1 from the initial electrode 2, particularly undertransient conditions when the arc is switched on and off, or duringsharp variation of its current value, etc.

The employment of this first direction of reducing the temperature ofthe initial electrode 2 is preferable in electric arc treatment with abar-shaped solid electrode, e.g. electric welding, electric-arc buildup,deposition by spraying, etc.

The second method of reducing the temperature of the initial electrodeconsists in reducing the density "q" of the heat flow into the trueelectrode 1, i.e. increasing the working surface area F of the trueelectrode. This method is preferable in electric arc treatment with ahollow electrode made of the same materials as the solid electrodedescribed hereinabove, via a channel into which either the entire amountof the plasma-forming mixture or a part thereof is supplied, e.g. intreatment of metallurgical melts, in gas-heating, etc.

EXAMPLE 1

Plasma welding of steel plate pieces 0.3 cm thick was conducted alongraised edges in carbon dioxide supplied at a rate from 0.4 m³ /hr to 0.6m³ /hr. The welding was performed at a 195 A d.c. current with anoutward arc, in a plasmatron with a composite water-cooled electrodecomprising a copper holder 3 having press-fitted therein the initialelectrode (cathode) 2 of a hafnium bar of a diameter d₂ =0.05 cm, wih acurrent density at the initial electrode 2 equalling 9.934×10⁴ A cm².The welding was conducted with constant restoration of the true carbonelectrode 1, i.e. with its unvarying dimensions and geometry forpractically unlimited time, which proved that the temperature in thezone of contact of the true and initial electrodes 1 and 2 was below thetemperature of sublimation of carbon.

When the welding current was raised to 210 A, which with the givendiameter d₂ of the active insert equalling 0.05 cm² corresponding to theraising of the current density thereat to 1.07×10⁵, i.e. to a valueexceeding by but 6.9% the abovestated upper limit of the currentdensity, each time when the arc was turned off, the true electrode 1separated itself from the initial electrode 2. This would not providefor a prolonged operation of the electrode in a constant restorationmode.

EXAMPLE 2

Mechanized plasma-jet spraying of a refractory protecting lining ofcorundum (aluminum oxide) onto the surface of panels of silicon carbidewas conducted. A mixture of natural fuel gas and carbon dioxide was usedwith the supply rates of 2.0 and 6.0 m³ /hr, respectively. The sprayingwas performed at 500 A d.c. in a plasmatron with a composition electrode(cathode) mde of the water-cooled copper holder 3 having press-fittedtherein a graphite bar-type initial electrode 2 of a 0.12 cm diameter,with the current density at the initial electrode equalling 24.4×10⁴A/cm². The electrode performed in a constant restoration mode forseveral months.

EXAMPLE 3

A slag bath was heated and blasted for reduction of slag components witha blast of a gas heated in a d.c. electric arc ignited between anannular graphite electrode-cathode and the slag melt-anode at a 1100 Acurrent. The gaseous atmosphere of the arc was a mixture of natural fuelgas and carbon dioxide supplied at respective rates of 10 m³ /hr and 5m³ /hr via the axial passage 25 mm in diameter in the initial graphiteelectrode 2 having an outside diameter d₂ =125 mm.

Under these conditions, there was being formed on the face portion ofthe initial graphite electrode 2 a true carbon electrode 1 whichperformed in a mode of constant restoration at a temperature in the areaof contact between the initial and true electrodes 1 below the point ofsublimation of carbon.

INDUSTRIAL APPLICABILITY

The present invention can be employed at plasma treatment of metals,e.g. their welding, buildup, spraying, and also in treatment of metaland slag melts in metallurgy.

I claim:
 1. In a method of constantly restoring an electrode duringplasma treatment of materials, in an electric arc device having a pairof spaced electrode elements, one of said electrode elements beingconstantly restored and comprising a water cooler holder (3) supportinga solid bar-shaped electrode (2) forming an initial electrode onto whichis deposited a cathode electrode (1) forming a true electrode, thecathode electrode (1) being deposited in a thin layer onto the surfaceof said solid bar-shaped initial electrode; the materials being treatedby introducing, into the space between the electrode elements and on theinitial and cathode electrodes themselves, a plasma-forming mixtureincluding carbon-containing compounds chosen from the class ofhydrocarbons and carbon oxides, which provides, in the course of arcburning, carbon deposition upon the initial electrode (2), said initialelectrode being made of either carbon or carbide-forming metals, andformation on the surface of said electrode of a true carbon cathode, andconducting the treatment at a temperature of contact of said initialelectrode (2) and the cathode electrode (1) at a temperature notexceeding the temperature of sublimation of carbon which is about 4000°K.
 2. A method according to claim 3, wherein the treatment with thesolid initial electrode (2) is conducted at a current density at theinitial electrode (2) within a range from 10⁴ A/cm² to 10⁵ A/cm².
 3. Themethod of claim 1, wherein the carbon electrode (1) is preferably formedof graphite.
 4. The method of claim 1, wherein the carbide-formingmetals include hafnium, titanium, tungsten, molybdenum and zirconium. 5.The method of claim 1, including conducting the process in the spacebetween the electrodes on the surface of the initial electrode (2), andsaid cathode electrode (1) each act as a true electrode, and saidinitial electrode (2) acts as the working electrode within the initialperiod.
 6. The method of claim 1, wherein the plasma-forming mixture isselected from the group consisting of hydrocarbons in their initialgaseous state, hydrocarbons in their liquified state, and blendsthereof, and carbon monoxide and carbon dioxide.
 7. The method of claim1, wherein the plasma-forming mixture includes methane.
 8. The method ofclaim 1, wherein the plasma-forming mixture includes acetylene.
 9. Themethod of claim 1, wherein the plasma-forming mixture includes propane.10. The method of claim 1, wherein the plasma-forming mixture includesbutane.
 11. The method of claim 1, wherein said bar-shaped electrode (2)is press-fitted into said water cooler holder (3).
 12. The method ofclaim 1, wherein the initial electrode (2) is formed from eithertungsten or molybdenum or compounds thereof in argon.
 13. In a method ofconstantly restoring an electrode during plasma treatment of materials,wherein the materials are electrically arc treated in an electric arcdevice having a pair of spaced electrode elements, one of said electrodeelements being a constantly restored electrode, and comprising a watercooler holder (3) supporting a solid bar-shaped electrode (2) forming aninitial electrode (2) during the initial period onto which is depositeda cathode electrode (1) forming a true electrode, the cathode electrode(1) being deposited in a thin layer onto the surface of said solidbar-shaped electrode (2) which acts as a working electrode within aninitial period upon first striking of an arc until the true electrode(1) is formed of carbon deposited from the carbon-containing gaseousatmosphere of the arc, the materials being treated with constantlyrestored solid and hollow electrode by introducing, into the spacebetween the electrode elements and on the initial and cathode electrodesthemselves, a plasma-forming mixture including carbon-containingcompounds chosen from the class of hydrocarbons and carbon oxides, whichprovides, in the course of arc burning, carbon deposition upon theinitial electrode (2), said initial electrode (2) being made of eithercarbon or carbide-forming metals, and formation on the surface of saidinitial electrode (2) of a true carbon cathode, and conducting thetreatment at a temperature of contact of said initial electrode (2) andthe cathode electrode (1) at a temperature not exceeding the temperatureof sublimation of carbon.
 14. In the method of claim 13, wherein thetemperature of sublimation of carbon is approximately equal to (≅) 4000°K.
 15. In the method of claim 13, wherein after the initial stages ofthe process, said initial electrode (2) becomes a passive elementfunctioning in the same manner as said copper water cooled holder (3),solely as one of the links of the heat transfer chain from the workingsurface of the true electrode (1) to the cooling medium.
 16. In themethod of claim 13, wherein the temperature of the initial electrode isreduced by reducing the density of the heat flow into the true orcathode electrode (1).